1. Field of the Invention
The invention relates to an evaluation unit to evaluate detector events registered by one or more detectors, whereby the evaluation unit contains signal processors that serve to process signals from the detectors.
2. Description of the Related Art
An evaluation unit of this type is known from the article titled xe2x80x9cDigital Centroid-Finding Electronics for High-Rate Detectorsxe2x80x9d by P. J. Pietraski, Z. Zojceski, D. P. Siddons, G. C. Smith and B. Yu, Brookhaven National Laboratory, Upton, N.Y. 11973, United States, 1998 in the IEEE Nuclear Science Symposium. With this detector system, individual detectors of a detector system are combined to form segments. Each segment has a digital signal processor that processes the signal.
A two-dimensional, position-resolving detector for subthermal neutrons is known from prior public use at the Julich Research Center. Here, the physical conversion of neutrons into light takes place in a 1-mm thick 6Li-glass scintillator. The read-out of the 8xc3x978 photomultiplier and the determination of the position of the event are done with a coarse-fine method by means of a computer specially designed for this purpose using tabular evaluation. The maximum counting rate of the system currently lies at 100 kHz.
Continuous improvement of the beam control has made it possible to increase the beam intensity at the sampling position and thus also the counting rate of the scattered neutrons on the detector. As a consequence, lag time effects occur with many samples that scatter strongly. On the other hand, in the case of samples that only scatter slightly and with correspondingly longer measuring times, the xcex3-background has a negative effect.
The known detector system, as described in Nuclear Instruments and Methods A305 (1991), pages 423 through 432, has the following values:
The xcex3-sensitivity is estimated by means of the following measurements:
background of the detector caused by cosmic radiation, scintillator effects, etc.: approximately 12 to 14 cmxe2x88x922hxe2x88x921;
the same detector installed in a radiation housing and in a detector tube: approximately 6to 7 cmxe2x88x922hxe2x88x921;
xcex3-background in the case of irradiation with a 60 Co source (1.3 MeV-xcex3) approximately 4 mrem hxe2x88x921 on the detector surface: approximately 35 to 53 cmxe2x88x922hxe2x88x921. The dose rate measured with a dosimeter corresponds to a xcex3-current density of 2xc3x97103 xcex3cmxe2x88x922sxe2x88x921 or 72xc3x97105 xcex3cmxe2x88x922hxe2x88x921. Consequently, the xcex3-sensitivity of the detector is approximately 1xc3x9710xe2x88x925.
xcex3-background in the measurement using neutrons, whereby approximately half of the detector surface area is covered with a 1-mm cadmium plate: approximately 53 to 64 cmxe2x88x922hxe2x88x921.
The event of an absorbed neutron is represented by its signals generated on the 64 photomultipliers (PM). These signals, most of which consist merely of electronic noise, are used to form the address of a memory position whose content is then increased by 1.
The storage should be done according to the position where the neutron absorption has taken place. This is only possible to a limited extent since the available PM signals Sik are stochastic quantities that fluctuate around their anticipated values Bik (x, y) and thus do not constitute clear-cut functions of the position. Only their anticipated values are well-defined functions of the position of the neutron absorption. In principle, the position cannot be determined any more precisely than by searching for the most likely position for the generated PM signals. For this purpose, it is necessary to know the anticipated values of all of the multiplier signals as a function of the event positions (x, y). The memory addresses then become identicalxe2x80x94except for a scale factor and an offset valuexe2x80x94to the most likely positions ascertained (xw, yw). An event is then stored in that the content of the memory cell is increased by 1.
The invention has the objective of creating an evaluation unit of the generic type by means of which the signal events can be reliably detected and quickly further processed.
According to the invention, this objective is achieved in that an evaluation unit of the generic type is configured in such a way that it contains at least one distributor that relays the signals from the detectors to at least one selected signal processor.
The evaluation unit can be configured in different ways. In particular, it is possible to design the evaluation unit in such a manner that its components are optimized for performing one or more prescribed evaluation processes.
For instance, it is advantageous to equip the evaluation unit with analog output channels, preferably also analog pulse shapers, and to provide analog-digital transducers subsequent to the pulse shapers.
The above-mentioned arrangement allows the signal to be detected especially quickly, as is desired in online operations. The invention, however, also encompasses embodiments in which the evaluation unit is completely digital or completely analog, or else it has a different division between analog and digital components.
Especially fast and reliable signal processing can be achieved in that the distributor is a digital module. The distributor is preferably an Application Specific Integrated Circuit (ASIC). In particular, this module is a digital gate array, preferably a Field Programmable Gate Array (FPGA). A Field Programmable Gate Array entails the special advantage of being freely programmable.
The system is suited for any counting rates and is also well-suited for high counting rates in the range from 1 MHz to several MHz.
The evaluation unit and the detector system equipped with such a unit are especially suited for a large number of channels, for example, 256xc3x97256 channels. Such a large number increases the resolution for partial sectors (subsectors).
The detection and/or evaluation speed can be increased by configuring the distributor in such a way that it relays a signal to be processed to a signal processor that has free processing capacity.
In this manner, measurements can be carried out in real time.
Interference signals can be suppressed in that the evaluation unit has at least one module that performs a differentiation operation. This eliminates slow interference signals.
Another improvement of the suppression can be achieved if the module is a shaping amplifier.
An increase in the reliability of the data processing can be achieved in that the signal processors are digital signal processors.
In order to attain low-interference data transmission, it is advantageous for optical waveguides to be employedxe2x80x94at least in some areasxe2x80x94for the data transmission.
In order to be able to utilize the detector system for time-of-flight measurements, it can be configured in such a way that the evaluation unit can allocate radiation events to time-of-flight channels that are different from each other.
In order to permit detection in real time during the time-of-flight measurements and/or other time-critical applications, it is advantageous to provide memory banks having a data capacity of at least 24 bits. Memory banks with a data capacity of at least 32 bits are particularly advantageous.
Another advantage of various configurations of the detector system according to the invention is the possibility of integrating it into an existing software system for purposes of data transmission as well as of the start/stop, interruption/continuation of the measurement as a function of external parameters.
This is a future-oriented detector system that stands out for technologies involving pulse processing, for instance, digital signal processing and components adapted to such technologies.
The read-out electronic system can replace the existing electronic system, so that existing evaluation and control software can continue to be used. Moreover, it is also an autarchic system that is suitable for a wide array of experiments. Adaptation to an already existing system can preferably be done with an interface to an already existing electronic system. The system is scalable with respect to the computing capacity. For this purpose, it is advantageousxe2x80x94depending on the execution time needed for the algorithms usedxe2x80x94to employ one or more signal processors, especially digital signal processors (DSPs), whereby the number of signal processors to be used does not bring about any changes in the basic structure of the system.